After adding a mailbox to either a Mailbox or Calendar sync collection, the sync fails for that mailbox with the following error:
Microsoft.Exchange.WebServices.Data.ServiceResponseException: No mailbox with such guid
There are multiple causes of this Exchange Web Services (EWS) error.
Mailbox does not exist in Exchange Online
There are duplicate mailboxes in Exchange Online
The Primary SMTP address or UPN is associated with another mailbox in Exchange Online
License the mailbox for Exchange.
Remove any duplicated mailboxes for the impacted user.
Ensure only this mailbox has the Primary SMTP and UPN associated with it.
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